In AnymailInboundMessage, work around Python 2 email.parser.Parser's
lack of handling for RFC2047-encoded email headers. (The Python 3 email
package already decodes these automatically.)
Improves inbound handling on Python 2 for all ESPs that provide raw
MIME email or raw headers with inbound events. (Mailgun, Mandrill,
SendGrid, SparkPost.)
Work around Python 2 email.parser.Parser bug handling RFC5322 folded
headers. Fixes problems where long headers in inbound mail (e.g.,
Subject) get truncated or have unexpected spaces.
This change also updates AnymailInboundMessage.parse_raw_mime to use
the improved "default" email.policy on Python 3 (rather than the
default "compat32" policy). This likely fixes several other parsing
bugs that will still affect code running on Python 2.
Improves inbound parsing for all ESPs that provide raw MIME email.
(Mailgun, Mandrill, SendGrid, SparkPost)
